Page 179 - Introduction to Paleobiology and The Fossil Record

166 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

30
estimated total

asymptote 20
collector
curve Taxa (95% confidence)
New species 10

100 200 300 400 500 600 700
Effort (b) Specimens
(a)
Figure 7.3 (a) The classic collector curve showing the sigmoid (or logistic) shape of the curve of
cumulative new species plotted against effort (number of specimens collected/number of days
spent looking/number of investigators), with a rapid rise and then a tailing off to an asymptote.
(b) Rarefaction curve that shows the number of species likely to be identified from samples of a

particular size. (b, based on Hammer & Harper 2005.)

foraminifera species died out (Fig. 7.4). smearing of the record happens, and it is now
However, should a paleontologist describe termed the Signor–Lipps effect in their honor
this as an example of catastrophic or gradual (see also p. 26). The Signor–Lipps effect can
extinction? A gradualist would argue that the make a sudden mass extinction seem gradual
extinction lasts for more than 0.5 myr, too (Fig. 7.5b).
long to be the result of an instant event. A These kinds of problems are especially
catastrophist would say that the killing lasted likely for organisms such as dinosaurs. Their
for 1–1000 years, and would argue that the bones are preserved in continental sediments,
stepped pattern in Fig. 7.4 is the result of which are deposited sporadically, and speci-
incomplete preservation, incomplete collect- mens are large and rare. Nevertheless, two
ing or reworking of sediment by burrowers. teams attempted large-scale fi eld sampling in
More precise dating and more precise assess- Montana to establish once and for all whether
ment of sampling problems are needed to the dinosaurs had drifted to extinction over
sharpen the defi nitions. 5–10 myr, the view of the gradualists, or
The rock record can be misleading (see p. whether they had survived at full vigor to the
70), and gradual extinctions might look cata- last minute of the Cretaceous Period, when
strophic and catastrophic extinctions gradual they were catastrophically wiped out. Need-
(Fig. 7.5). If there is a gap in the rock record, less to say, one team found evidence for a
especially at a crucial time line such as the KT long-term die-off, and the other team demon-
boundary, species ranges are cut off artifi cially strated sudden extinction.
and the pattern looks sudden (Fig. 7.5a). The The problem was not that either team had
opposite effect, an apparently gradual pattern, done their work badly, but that the fossils
can happen because paleontologists will never were still too scattered, and the dating of the

find the very last fossil of a species. Phil Signor rocks was not good enough, to be sure. Geol-
and Jere Lipps showed how this backward ogists work in millions of years, and yet